For everybody who looks back at some computer or computer program from the past with a certain nostalgia, there are emulators. These are smart programs that can bring a computer from the past back to life on a modern-day computer.
An example. Let's say, as a kid you occasionally visited a friend who had an Atari 2600 games console. And nothing was more fun than playing a few games of PacMan of Space Invaders with that thing.
That was years ago. This friend has moved a long time ago, you never saw the games machine ever again. Nowadays, you have a fast PC with quite some games on it. Some of them even resemble PacMan and Space Invaders, but it's never quite the same. Wouldn't it be fun to see and hear those true original games just one more time?
Well, no problem! Now you can play all of those games on an ordinary PC, without need of the old hardware. On the internet, look for the right emulator (in our example an Atari 2600 emulator). Download this program, run it, and your PC is temporarily turned into the computer of your choice.
Now all you need are the original games (PacMan, Space Invaders). These used to be on cartridges, tapes or disks. In general, the PC cannot cope with these media. Just try putting a cartridge for an Atari in your PC. Most of the time you must have access to the original hardware to port the games from their old media to the PC. And even then it will take some effort. It is easier to go looking on the internet once more. More often than not, someone has already taken the effort of porting the programs in the right way. Finding the game generally means you can download it as a single file. The emulator will recognise the file as if it was on the original medium, and off you go playing the game.
I have taken some old games console as an example. But there are emulators for many more types of computers. Home computers, old mini computers, videogames from the arcade, programmable calculators, organisers. For almost every computer I had ever heard of, there is someone who has written an emulator for it. Often there is even more than one around. Then it is just a question of trying them out and choosing your favourite.